(1) Field of the Invention
This invention relates to a vaccine for treatment or prevention of Acquired Immune Deficiency Syndrome (AIDS) or pre-AIDS (also known as AIDS-Related Complex or ARC), including novel antibodies and antisera to thymosin .alpha..sub.1 peptide fragments and analogs thereof, or to peptide fragments of the p17 gag protein of HTLV-III, LAV, ARV-2 or other similar retrovirus. This invention also relates to the novel peptide fragments and analogs thereof, per se. This invention also relates to a diagnostic test for directly detecting the presence in the blood of the HTLV-III, LAV, ARV-2, etc, retroviruses.
(2) Discussion of the Prior Art
Acquired immune deficiency syndrome (AIDS) is a disease which is characterized by a decrease in helper T-cells (T.sub.4.sup.+) and T.sub.4.sup.+ /T.sub.8.sup.+ lymphocyte ratios, an increase in the incidence of opportunistic infections and progressive paralysis of the immune system. The etiologic agent of AIDS has been identified as a human T-lymphotropic retrovirus, HTLV-III (also termed LAV or ARV). In many respects, the clinical symptoms of AIDS are indistinguishable from symptoms often seen in children with rare primary immunodeficiency (PID) diseases associated with thymic aplasia or hypoplasia and an increased susceptibility to opportunistic infections, including pneumocystis carinii pneumonia. The possible involvement of the thymus gland, which plays a key role in the maturation and function of the lymphoid system, in AIDS has been implicated by the detection of elevated levels of a T.alpha..sub.1 -like peptide in the blood of individuals with AIDS or belonging to the AIDS risk group. See Naylor, P. H. et al, NYAS Monograph on AIDS, 437, 88 (1985); Hirsh, E. M. et al. New Eng. J. Med., 308, 45 (1983); Biggar, R. J. et al, New Eng. J. Med., 309, 49 (1983); Kreiss, J. K. et al, Annal Int. Med, 100, 178 (1984); Kessler, C. M. et al, Brit, J. Hematology, 58, 325 (1984).
T.alpha..sub.1 was the first thymic hormone purified to homogeneity and sequenced from the partially purified thymosin fraction 5 (TF 5). See Goldstein, A. L. et al., Proc, Natl, Acad, Sci., U.S.A., 74, 725 (1977) and U.S. Pat. Nos. 4,002,740; 4,010,148; 4,148,788 and others. It is an acidic polypeptide (pI 4.2) with a molecular weight of 3108, has many of the biological activities of thymosin fraction 5 (TF 5) and is a potent immunomodulator in vivo and in vitro. T.alpha..sub.1 acts primarily on helper T-cells and has been found in animals and humans to enhance the expression of T-cell markers and to stimulate the production of a number of lymphokines, including interleukin-2 (IL-2) and .gamma.-interferon, and to restore immune function and tumor immunity. T.alpha..sub.1 is currently the first biological response modifier (BRM) which has been shown to restore helper T-cell function and prolong survival in patients with lung cancer following radiotherapy, Schulof, R. S., et al., J. Biol. Resp. Mod. 4, 147 (1985). U.S. Pat. No. 4,521,407 to Pelham, et al. discloses that a particular calf thymus extract can stimulate the cellular immune system of a person having AIDS.
Given the clinical similarities between AIDS and PID's in children, it was expected that T.alpha..sub.1 levels would be depressed in patients with AIDS or AIDS Related Complex (ARC). T.alpha..sub.1 can be measured by radioimmunoassay as described in U.S. Pat. Nos. 4,264,571 and 4,339,427. Surprisingly, examination of the blood from patients with AIDS and ARC shows markedly elevated levels of T.alpha..sub.1. For example, 44 of 72 individuals with Kaposi's sarcoma (60%) and 12 of 22 individuals with Pneumocystis carinii pneumonia (54%) had T.alpha..sub.1 levels above two standard deviations from the mean of normal individuals. Subsequent studies confirmed the initial findings for a nationwide sampling of patients with AIDS, which included homosexuals and bisexuals, intravenous drug abusers, Haitians, and homosexuals with lymphadenopathy.
Three possible alternatives have been proposed to account for the elevation of T.alpha..sub.1 : (1) a defect in helper T-cells resulting in increased levels of biologically active peptide, i.e. end-organ failure; (2) a viral invasion of the thymus epithelial cells with subsequent loss of control over hormone production and/or processing; or (3) the presence of a biologically inactive but immunologically cross-reactive protein related to the "AIDS" virus. As reported by Naylor, P. H., et al., AIDS (Alan R. Liss, New York) (1985) (p. 265), further chemical and immunochemical characterization of the T.alpha..sub.1 -like peptide in the blood of homosexuals at high risk for AIDS indicate that this abnormally elevated peptide may be a cross-reacting molecule and that, in fact, authentic T.alpha..sub.1 is often significantly decreased.
In order to identify the T.alpha..sub.1 cross-reacting substances, the present inventor initiated a comprehensive search for homologous peptides using computer-assisted analysis. Over 3,500 protein sequences at the sequence bank of the National Biological Research Foundation in Washington, D.C. and the catalogue of available HTLV-III/LAV and other retroviral isolates from the laboratory of Dr. Robert Gallo at the National Cancer Institute (NCI), were used. Although no significant homologies with other thymic hormones or lymphokines were found, it was unexpectedly discovered that T.alpha..sub.1 and the gag (core) proteins (p17) of HTLV-III/LAV and ARV-2 share a 44% to 50% homology in an 18 amino acid region (positions 11-28 on T.alpha..sub.1 and 92-109 on the p17 gag protein) of their primary sequences.
Currently, essentially all ongoing research strategies for the development of vaccines against diseases induced by HTLV-III/LAV/ARV gene and its products (i.e. proteins and/or glycoproteins), are focussing on the envelope gene, env. See, for example, Fischinger, P. J., et al., "Current Status and Strategies for Vaccines against Diseases Induced by Human T-Cell Lymphotropic Retroviruses (HTLV-I, -II, -III)" Cancer Research, 45 4694s-4699s (Sept. 1985); L. Davis, "AIDS Vaccine Research: Promising Protein", Science News, 129, 10, p. 151 (Mar. 8, 1986). Presumably, this strategy is based on the conventional wisdom that the envelope proteins (i.e. outer coat) are most likely to provide the antigenic site(s) which could be "recognized" by the body's immune system leading to the production of antibodies.
However, as pointed out, for example, by Fischinger, et al., due to the extensive heterogeneity and antigenic variations observed among various HTLV-III/LAV isolates in the env gene and corresponding env proteins, development of vaccines with these immunogens is expected to encounter considerable difficulty and may possibly be futile.
In contrast, the gag core proteins of HTLV-III/LAV, although not apparently "exposed" to the immune system defenses, are much more highly conserved and greatly less subject to mutation and genetic drift.
The present invention is based on the quite surprising discovery that antigenic determinants common to gag proteins of HTLV-III/LAV are readily accessible to killing with antibodies directed against specific epitopes, namely the epitopes found to have a high of degree of homology with thymosin .alpha..sub.1.
Robert C. Gallo, et al. in U.S. Pat. No. 4,520,113 describe a diagnostic test for the serological detection of antibodies to HTLV-III in sera of patients with AIDS and ARC conditions. Such diagnostic procedure which is currently in wide practice suffers from various defects leading to false results. Specifically, a diagnostic test which determines the presence of antibodies to HTLV-III/LAV viruses has the following drawbacks which could result in false-negative or false-positive results. Some patients who are exposed to the AIDS virus may have deficient immune responses and not produce any antibodies. Some patients may have been recently exposed but not yet produced any or detectable amounts of antibody (antibodies to AIDS often take up to 8 weeks to be produced in detectable levels). Some patients who have antibodies in their sera may not have any virus remaining in their system. Thus, a "false-positive" result would be given in the latter case while "false-negative" results would be suggested in the two former cases.
Clearly, therefore, a diagnostic test which detects the presence of the AIDS virus directly and selectively would be of extreme importance for the early detection and diagnosis of patients exposed to or in a high risk category for contracting AIDS.